Eukaryotes

1. MULUNGUSHI UNIVERSITY
SCHOOL OF MEDICINE AND
HEALTH SCIENCES
BIOMED II
Control of Gene Expression in
Eukaryotic cell
2/5/2024 -SP@MU2023- 1

2. Introduction
 Gene regulation is a fundamental process that
allows an organism to control the expression
of its genes, determining when and to what
extent specific genes are turned on or off.
 Only a portion of each cell's genes are
expressed, or turned on, at any given time.
 The process of turning genes on and off is
known as gene regulation.
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3. Introduction
 In contrast to prokaryotes, gene expression in
eukaryotes is controlled at multiple levels.
 Reason: This could be attributed to the cell
arctechture of the eukaryotic cell.
 Eukaryotic genes are not organized into
operons, so each gene must be regulated
independently
 Regulation of gene expression can happen at
any of the stages : at DNA packaging,
Transcription and translation into protein

4. Purpose of gene regulation
 In general :
 Allows the cell to express gene products (protein or
RNA) only when needed
 By differential expression of genes, cells can respond to
changes in the environment or adapt to new food
sources
 Gene regulation drives cellular differentiation and
morphogenesis in the embryo, resulting in the formation
of various cell types.
 Differential expression, allows cells to specialize in
multicellular organisms.
 is an important part of normal development. 4

5. • Cellular Differentiation: Gene regulation
plays a crucial role in the process of cellular
differentiation, where cells become
specialized for specific functions.
• Response to Environmental Changes:.
Cells can adjust their gene expression
patterns in response to external signals, such
as changes in temperature, nutrient
availability, or the presence of specific
molecules
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6. Development and Growth:
• Gene regulation is essential for the
coordinated growth and development of an
organism.
• It ensures that genes are activated or
repressed at specific times and in specific
tissues, allowing for the proper formation of
organs, tissues, and structures during
embryonic development and beyond
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7. Cell Cycle Control:
Genes involved in the cell cycle, including
those regulating cell division and apoptosis,
are tightly controlled.
• Proper regulation ensures the accurate
progression of the cell cycle, preventing
abnormal cell growth, and maintaining
genomic stability.
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8. Energy Conservation:
 Gene regulation helps conserve energy by
ensuring that cells only produce the proteins
they need at a given time.
 Unnecessary protein synthesis can be
energetically costly, and gene regulation
helps optimize resource allocation within
the cell.
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9. Level of gene regulation
 epigenetic,
 transcriptional,
 post-transcriptional
 translational
 post-translational
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10. Cell Membrane
Nucleus
Transcription
(RNA polymerase II)
DNA
hnRNA
Post transcriptional
Processing – Capping,
Splicing, Polyadenylation
AAAAAAAA
mRNA
Cytolasm
AAAAAAAA
Translation
Protein
Post – translational
modifications
Protein product

11. DNA Packaging
 Each chromosome contains a single DNA molecule that
extends from one end to the other.
 DNA is coiled around a protein( histones) to form nucleosomes
 A "chromatin" is formed when a DNA molecule is coiled and
folded multiple times with associated proteins.
 The chromosomal proteins are divided into Histone proteins
and Non-Histone proteins.
 Histone proteins are positively charged and have many arginine
and lysine amino acids that bind to the negatively charged
DNA. Histones are of two types:
 Core Histones (H2A, H2B, H3 and H4)
 Linker Histones (H1)
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12. Cont...
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13. The Normal Human Chromosomes
• Normal human cells contain 23 pairs of homologous
chromosomes ie 22 pairs of autosomes and 1 pair of sex
chromosomes.
• Sex chromosomes are XX in females and XY in males.
• Both X are homologous. Y is much smaller than X and
has only a few genes
• Therefore, Mammalian females inactivate one X
chromosome as a form of dosage compensation to
equalize X-chromosome expression in both sexes.
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14. Epigenetic Mechanisms of Gene Regulation
• epigenetics is the study of heritable
changes in gene expression that occur
without a change in the primary DNA
sequence of an organism.
• Epigenetics: Occurs when a chemical
compound or protein attaches to the gene
and alters gene expression. The DNA
sequence is not changed.
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15. Epigenetic Mechanisms of Gene Regulation
 Epigenetic regulation is mainly executed by DNA
methylation and histone modification
 Chromatin Structure Affects Gene Expression
Euchromatin: Loosely packed DNA
Heterochromatin: tightly packed form of DNA
or condensed DNA
 Chromatin structure is affected by a wide variety
of modifications to histones as well as DNA
methylation
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16. DNA methylation
 DNA methylation was the first modification of
chromosome structure shown to act epigenetically.
 The addition of a methyl group to cytosine by a
methylase enzyme creates 5-methylcytosine, but
this change has no effect on its base-pairing with
guanine.
 Methylation is a way of marking genes for
silencing
 High levels of DNA methylation correlate with
inactive genes, and the allele specific gene
expression seen in genomic imprinting is mediated
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17. • 5-Methyl-cytosine is the only modified base
commonly found in eukaryotes.
Caenorhabditis elegans, Drosophila, and
yeast, however, contain little or no 5-
methyl-cytosine
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18. Cont..
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DNA methylation. Cytosine is methylated, creating 5-methylcytosine.
Because the methyl group (green) is positioned to the side, it does not
interfere with the hydrogen bonds of a G–C base-pair, but it can be
recognized by proteins.

19. Histone modification
 Histones are chromosomal proteins that tightly wind
DNA so that it fits into the nucleus of a cell.
 If a gene is to be transcribed, the nucleosomes around
DNA can slide down to open that specific chromosomal
region and allow access for RNA polymerase and other
proteins, to bind to the promoter region and initiate
transcription.
 Conversely, in closed configuration, the RNA
polymerase and transcription factors do not have access to
the DNA and transcription cannot occur
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20. Cont...
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 Since DNA negatively charged, changes in
the charge of the histone will change how
tightly wound the DNA molecule will be.
 Histone Modifications include acetylation
and methylation of lysine; and
phosphorylation of serine, threonine, and
tyrosine

21.  Addition of phosphate, methyl, or acetyl groups
acts at signal tags that open or close a
chromosomal region
 E.g acetylation, especially of H3, is correlated
with active sites of transcription, both in
regulatory regions and in the transcribed region
of the gene itself.
 While methylation of the same histone (H3)
can have the opposite effect, depending on the
lysine methylated.
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22. Cont...
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A) When nucleosomes are spaced closely together, transcription
factors cannot bind and gene expression is turned off. (B) When
nucleosomes are spaced far apart, transcription factors can bind,
allowing gene expression to occur.

23. Transcriptional Control of Gene Expression
• Transcriptional regulation is control of whether or
not an mRNA is transcribed from a gene in a
particular cell.
• In eukaryotes, RNA polymerase alone cannot initiate
transcription as it requires other proteins, or
transcription factors, to facilitate transcription
initiation
• Transcription factors are proteins that bind to the
promoter sequence and other regulatory sequences
to control the transcription of the target gene
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24. cont...
 Transcriptional factors could be general transcription
factors and specific transcription factors.
 General factors are necessary for the assembly of a
transcription apparatus and recruitment of RNA
polymerase II to a promoter. Eg Transcription factor
RNA polymerase II (TFII).
 Specific factors increase the level of transcription in
certain cell types or in response to signals.
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25. Structure of a Eukaryotic gene
Exons – Coding
Introns – Non coding
Promoter – Essential for transcription
Enhancer – modulates the rate of transcription
TATAA –Basal transcription complex
CAAT - NF1
GC – Sp1
Oct – Octamer binding protein
INR – Binds subunits of TFIID
EXON 1 EXON 2 EXON 3
Intron 1 Intron 2
TATAA
Promoter
Poly-adenylation
signal
~100bp
Enhancer
Start site for
transcription
CAAT
GC
Oct
INR

26. Promoter
 the promoter region is immediately upstream of the
coding sequence. This region can range from a few to
hundreds of nucleotides long.
 The purpose of the promoter is to bind transcription
factors that control the initiation of transcription
 Within the promoter region, resides the TATA box
which is a repeat of thymine and adenine dinucleotides
which binds transcription factors to assemle an
initiation complex.

27. Cont...
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28. Enhancers and Repressors
 Enhancers are binding sites for activators and in
some eukaryotic genes, there are regions that
help increase transcription.
 Transcriptional repressors can bind to promoter
regions and block transcription
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29. Post-transcriptional Control of Gene Expression
 Post-transcriptional regulation occurs after the
mRNA is transcribed but before translation
begins.
 This regulation can occur at the level of mRNA
processing, transport from the nucleus to the
cytoplasm, or binding to ribosomes.
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30. Alternative RNA splicing
 Post-transcriptional regulation occurs after the
mRNA is transcribed but before translation begins
 when introns are removed from the primary RNA
transcript by RNA splicing, the remaining exons
are spliced together to generate the final, mature
mRNA
 Alternative RNA splicing is a mechanism that
allows different combinations of introns, and
sometimes exons, to be removed from the primary
transcript
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31. Cont...
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Alternative RNA splicing
1 2 3 4

32. CONT...
 The lenghth mRNA , and its poly-A tail are
important for mRNA
 the binding of RNA-binding proteins (RBP) to pre
RNA can increase or decrease the stability of an
RNA molecule, depending on the specific RBP that
binds.
 The microRNAs, or miRNAs, can also bind to the
RNA molecule and further degrade i.
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33. microRNAs, or miRNAs
 miRNAs are short (21–24 nucleotides) RNA molecules that
are made in the nucleus and then chopped into mature
miRNAs by a protein called dicer.
 Produced miRNA is loaded into a complex of proteins
called an RNA-induced silencing complex, or RISC.
 The RISC includes the RNA-binding protein Argonaute
(Ago), which interacts with the miRNA.
 One of the the complementary strand is removed by
nucleases enzymes
 The the RISC is targeted to repress the expression of other
genes based on sequence complementarity to the miRNA
 the other RNAs are vital to the process of gene silencing
and participate in the mechanism of gene regulation, referred
to as RNAi or RNA interference
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34. Translational Control of Gene Expression
 Translation can also be regulated at the level of
binding of the mRNA to the ribosome.
 Ribosomes are found in cytoplasme and on the
endoplasmic reticulum (ER).
 Proteins destined to ER , use a signal sequence for
their transportation to ER and translation
resumes from there
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35. Post-translational Control of Gene Expression
 This type of control entails altering the protein after
it has been created in order to change its activity.
 enzyme inhibition
 The activity and/or stability of proteins can also be
regulated by adding functional groups, such as
methyl, phosphate, or acetyl groups.
 tagged proteins for degradation are moved to a
proteasome, an organelle that degrades proteins
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36. Genomic imprinting
• Genomic imprinting is an epigenetic
phenomenon that results in the expression
of genes in a parent-of-origin-specific
manner.
• In other words, the expression of certain
genes depends on whether they are
inherited from the mother or the father.
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37. • Cells normally have two copies, or “alleles,”
of autosomal genes on chromosomes other
than the X and Y.
• One allele is inherited from the mother
(maternal allele) and one is inherited from the
father (paternal allele).
• For most genes, both copies are expressed by
the cell.
• A small class of genes shows “monoallelic”
expression
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38. • In genomic imprinting, selection of the active
allele is nonrandom and based on the parent of
origin
• For example, a gene that is imprinted to be
expressed only when inherited from the father
will be silent if inherited from the mother, and
vice versa.
• Genomic imprinting affects a small subset of
genes and results in the expression of
• those genes from only one of the two parental
chromosomes.
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39. Examples of genomic imprinting in humans is
the gene
• One well-known example of genomic
imprinting in humans is the gene for insulin-
like growth factor 2 (IGF2), which is only
expressed when inherited from the father,
while the maternal copy is silenced.
• Conversely, another gene called H19, located
adjacent to IGF2, is expressed only when
inherited from the mother, with the paternal
copy being silenced.
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40. Genomic imprinting and neurodevelopmental
disorders
• Three neurodevelopmental disorders,
Prader–Willi syndrome,
• Angelman syndrome, and Rett syndrome
(all named after the physicians who first
described the disorders), are the result of
either direct or indirect deregulation of
imprinted genes.
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